Some of it may be cautious attitudes re: the next few decades on Earth. Climate change may make it even more competitive to survive in areas where the weather

Some of it may be cautious attitudes re: the next few decades on Earth. Climate change may make it even more competitive to survive in areas where the weather is pleasant. People are already being displaced by climate change. Horrible for the displaced people, and may contribute to crowding somewhere else. Increasing numbers of people and increasing expectations for material wealth may lead to more frequent and widespread conflicts over resources. It's possible I'll get to live a full life before things get too bad, but I'd only be adding to the troubles by bringing more humans onto the planet. If we can get fertility rates low enough, and find ways to adapt economic systems to cope gracefully with declining populations, maybe our troubles can be mitigated.